The present invention generally relates to reverb generators and more particularly to a reverb generator including a phase shifter or so called all-pass filter for applying a dispersion to an input audio signal spectrum.
Reverb generators are used in electric acoustic systems such as an electric musical instrument or a sound reproducing system for providing reverberations to the reproduced sound, or for enhancing the presence such that a listener feels as if he or she is listening to the reproduced sound in a concert hall or the like.
Conventional reverb generators typically comprise a delay circuit for delaying an input audio signal irrespective of the frequency and a feed back path including an attenuator for feeding back an output signal of the delay circuit to an input side thereof with a predetermined attenuation In the past, reverb generators used a tape recorder or a mechanical resonator as a delay means. In recent years, digital circuits are commonly used for this purpose.
A typical reverb generator produces a series of exponentially attenuating output impulses repeatedly responsive to a single input impulse with a predetermined interval of .DELTA.T which is specified by the delay time of the delay circuit. The attenuation of the output impulses is determined by the attenuating constant of the attenuator which controls the feed back ratio of the feed back path.
Such a conventional reverb generator has only two variable parameters for adjusting the reverberation, i.e. the attenuating constant of the attenuator and the delay time of the delay circuit Thus, there is a problem that the degree of freedom in the sound processing is limited. Further, there is a more serious problem in such a conventional reverb generator that an unnatural reverberation is generated when the feed back ratio and/or the delay time is increased in order to achieve a long sustaining reverberation or an enhanced presence as is realized in the actual concert hall. In an extreme case, the individual reverberations can be resolved by human ears and the individual reverberations cause an unpleasant feeling to the listener. Unless such an extraordinary effect is intentionally sought for, the range in which the attenuation constant and the delay time can be varied is extremely limited. As a result of this limitation, the achieved acoustic effect such as the presence of the natural and pleasant reverberation is correspondingly limited.
For example, if the delay time .DELTA.T exceeds about 30 msec, unnatural feeling becomes too conspicuous for actual use. Long sustaining reverberations caused by increasing the feed back rate similarly induce an unpleasant and unnatural acoustic effect. Thus, in the conventional feedback type reverb generator having an open loop transfer function of K.e.sup.-s..DELTA.T, the value of K specifying the feed back rate can not be chosen practically larger than 0.2-0.4. If one increases the value of K, the duration the reverb sustains is certainly extended but the undesirable effect such as the unnatural and unpleasant feeling or the distortion of the reverberation becomes conspicuous. In other words, the conventional reverb generator cannot fully exploit the advantageous feature of the feed back path which is potentially capable of developing a series of extremely long lasting and gradually changing reverberations repeatedly one after another by feeding back the generated reverberations.
Commonly owned U.S. patent application Ser. No. 111,075, a continuation of Ser. No. 867,234 filed on May 23, 1986 by Tominari, discloses simulation of a reverberation or so-called indirect sound in a concert hall by using an all-pass filter having a constant gain throughout the entire frequency range. The all-pass filter induces a frequency dependent time delay in such a manner that the time delay is large in a low frequency range and small in higher frequency range. In other words, the all-pass filter disclosed in the above U.S. patent application provides an electrical means for simulating the dispersion of the spectrum of the sound which takes place when the sound from a sound source is reflected by walls or floor of the concert hall. The conventional reverb generator lacks this capability of dispersion, and it is believed that this is the reason why the conventional reverb generators fail to produce the natural and pleasant long sustaining reverberations. It is known that a listener in the concert hall feels the presence as a result of the difference between the arrival time of a direct sound reaching the listener directly from the sound source and the indirect sound or reverberation caused by the reflections of the sound at the walls or floor of the concert hall. This indirect sound of course has a spectrum which is dispersed as already described.
In an actual concert hall, the sound wave radiated from the sound source is reflected repeatedly by the walls or the floor Thus, the indirect sound usually includes sound components produced by a plurality of reflections. Such a multiple reflection provides a feeling of dimension of the concert hall and is desirable for achieving the natural presence in the reproduced sound. The system and method described in the aforementioned U.S. patent application, though capable of producing a natural reverberation, cannot simulate the effect of such multiple or repeated reflections.